The present invention relates to an image recording method and apparatus for thermosensible paper, thermosensible film, thermographic paper, thermographic film and etc., and particularly relates to an image recording method and apparatus in which one pixel in the image portion consists of a plurality of micro-lines, so that the area gradations corresponding to the image density is performed.
In an image recording apparatus such as a character printer and an image printer, there is a printer of a type which heats the thermosensible material by a thermal head so as to form images, e.g. characters, and drawings. In case of forming images by use of the thermosensible material, although some images are sufficiently represented by one color (e.g. black), in order to reply to many requests of various expression, it is required to provide a thermal recording method accomplishing multiple-color recording having high contrast image.
As a conventional image recording method, a melting type thermal transfer recording method is provided, whereas the method heats an ink film from a reverse side thereof so as to transfer the melted or softened ink to a normal paper. This method is suitable to recording binary image such as letters, line drawings or the like.
The melting type thermal transfer recording method is disclosed in Japanese Patent Unexamined Publication No. Hei. 3-219969. In the method, a thermal head is provided with a heating element having a width in the record paper transferring direction (secondary scanning direction) narrower than a length in the head of a perpendicular direction (main scanning direction) thereto, the recording paper is transferred intermittently every number of units which correspond to the value of the width of the secondary scanning direction. During the transfer, each heating element is energized in accordance with the image data to change the recording area in one recording pixel so as to perform the gradation expression.
Hereupon, the above recording method uses the width of the heating element in the secondary scanning direction as the unit recording width. It is necessary, however, to narrow the width of the secondary scanning direction in order to improve the gradation of the recording pixel. The thermal head has a limit relating to production cost and durability, therefore, it is difficult to obtain a higher gradation.
Therefore, as an improved melting type thermal transfer recording method, a method is disclosed in Japanese Patent Unexamined Publication No. Hei. 4-19163. There is provided a melting type thermal transfer recording method to record an image providing higher gradation without particularly narrowing the width of heating element.
In the improved method, the thermal head is transferred intermittently every unit width relative to the recording paper, the unit width being narrower than the width of the heating element in the secondary scanning direction. The heating element is driven every unit width so as to change the recording width in the secondary scanning direction in accordance with the density of pixel thus recorded.
The heating element is driven every transferring unit width in accordance with the density of pixel thus recorded so as to change the recording width narrower than the width of the heating element in the secondary scanning direction. Accordingly, it may be performed to record an image providing higher gradation without particularly narrowing the width of the heating element.
As shown in FIGS. 4 and 5, the above melting type thermal transfer recording method forms pixel 1 including a plurality of micro lines 1a-1n in parallel to the secondary scanning direction, e.g. disposing the pixels formed to a square of 122 .mu.m side respectively into matrix formation of a pitch of 2 .mu.m, so as to form required gradation image 2.
While the gradation image 2 is formed, e.g. the heating elements of the thermal head corresponding to the micro lines 1a-1e are heated, and other micro lines 1f-1n thereunder is not heated. Namely, the heating element of the thermal head is driven and heated so as to heat the micro lines of each record pixel 1A, 1B, 1C, . . . in the order of secondary scanning direction and to increase the number of the micro lines heated in accordance with the density of the image 2, and the required image 2 is formed.
Accordingly, the number of the micro lines 1a-1n of each record pixel 1A, 1B, 1C, . . . is determined in accordance with the density of the image 2 so as to change an area ratio of a heated portion to an unheated portion. Thus, some pixels having different gradations from each other are combined so that the image 2 is formed as a completion having areal gradations including half-tone portions. Though a transferring resolution in the secondary scanning direction is therefore low, a gradational image is obtained.
However, the above conventional method of melting type thermal transfer record has the following problem.
In each record pixel 1A, 1B, 1C, . . . , some lines in the micro lines adjoining each other at the same one side of the pixels are heated, and other lines in the micro lines at another side of the pixel are not heated. The resolution of the image thus formed is relatively high.
In the case that characters are formed by the above recording, particularly one pixel unit in an inclined portion 3 of character as shown in FIG. 5 is represented by only black or only white, a difference 3a of inclined line in which the pixels record are continuously connected appears corresponding to at least one pixel. Therefore, the resolution when representing the character decreases, and a problem with respect to forming the character remains.